Steel parts, which are embedded in concrete, such as sectional steel girders or reinforcing inserts made of steel, are, in a concrete which is composed and worked in accordance with regulations, generally permanently protected against corrosion under a sufficiently thick concrete covering. This protection against corrosion does not depend upon the impermeability of the concrete, but on the alkalinity of the concrete interstitial water, which usually has a pH value &gt;12.5. Under these conditions, on the surface of the steel, a thin firmly adherent oxide layer is formed, which practically completely prevents corrosion. Only in this way does reinforced concrete become usable for external structural parts, which are exposed to weathering.
Under certain unfavourable conditions, however, the protection against corrosion of the concrete can be lost. Errors in construction as well as especially unfavourable environmental conditions are examples of such conditions.
The cause of the corrosion may be carbonatization of the concrete, which takes place when carbon dioxide from the air reacts with the alkaline components of the cement. As a result of such a chemical reaction, the pH value falls and the protection against corrosion is lost. Another cause of corrosion is the penetration of chlorides into the concrete, which can happen if, for example, de-icing salts are spread on the concrete roadway of a bridge, which then penetrate in solution into the inside of the concrete member. Both processes begin on the surface of the concrete and continue into the inside of the concrete to the steel parts embedded there, where they cause the oxide layer adhering to the latter to disappear. On the surface of the steel, a critical state is then reached, in which corrosion sets in on the steel parts, without this being discernible at first on the surface of the concrete. Corrosion damage only becomes discernible when the corrosion of the steel is already relatively well advanced and the concrete covering spalls because of the explosive pressure of the rust products. Generally the corrosion is by then so far advanced that very costly and expensive repair measures become necessary. In many cases, a repair is often no longer even possible, so that the structure has to be demolished and replaced with a new one. The extent of damage caused by corrosion of the reinforcement in reinforced concrete and prestressed concrete structures is relatively great. In roadbridge construction the repair work necessary each year today costs more than 1% of the replacement costs of the building material.